1. Field of the Invention.
The present invention relates to the structure of a multiple internal reflectance crystal.
2. Description of Related Art.
Several structures for multiple internal reflectance crystals are known in the art. For example, radiant energy, such as infrared energy, may enter a crystal through a first beveled face so as to reflect off a first side. The energy reflects between the first side and a second side down the length of the crystal by the physical phenomenon of total internal reflection. A sample that is placed against either the first or second side of the crystal selectively absorbs different frequencies of energy. The energy that is not absorbed exits the crystal through a second beveled face to a detector that measures the distribution of energy absorbed by the sample so as to obtain its spectrum.
The depth to which the incident energy penetrates a sample depends on the refractive index of the sample and the multiple reflectance crystal, as well as the angle of incidence at which the energy reflects off of the side of the crystal that is in contact with the sample. The angle of incidence may be changed by changing the angle at which energy enters the crystal, henceforth referred to as the entrance angle. A multiple internal reflectance crystal, however, introduces chromatic aberration into the resulting distribution of energy if the incident energy is not normal to the surface of an entrance face of the crystal. Therefore, each desired angle of incidence requires a separate crystal having an entrance face with an appropriate bevel angle. Changing crystals, however, consumes much time in realigning the transfer optics so that the energy both enters and exits the new crystal normal to the entrance face.
Another form of multiple internal reflectance crystal uses a prism to reflect the incident energy to the multiple internal reflectance crystal. The radiant energy is introduced normal to an entrance face of the prism. The prism reflects the radiant energy into the multiple reflectance crystal so that the radiant energy internally reflects off of a side of the crystal in contact with the sample. The prism, however, must be in physical contact with a lower surface of the multiple internal reflectance crystal to avoid introducing chromatic aberrations into the incident energy because the beam of radiant energy is not introduced into the crystal normal to the bottom face of the crystal. Radiant energy is collected from the crystal and prism in a similar manner. A prism- type crystal design does not require realigning optical components for each entrance angle. The requirement that the prism be in physical contact with the multiple internal reflectance crystal, however, means that the entrance face of the prism is always located a relatively short distance below the entrance face of the crystal. For many applications the multiple internal reflectance crystal must be located at a substantial distance from the source and receiver of the radiant energy. It is often difficult to introduce and collect radiation from the prism because the energy must enter the crystal substantially parallel to the crystal only a small distance below the sample surface.
Many other designs for multiple internal reflectance crystals are known in the art. No known design, however, permits any one of several multiple internal reflectance crystal having differing angles of incidence to be positioned at a location that is remote from a source and receiver of radiant energy without realigning the transfer optics. Moreover, most known crystal designs can receive the radiant energy only over a relatively small entrance angle.